added a transfer method to vm env

vm/environment.go

@@ -1,6 +1,7 @@
 package vm
 
 import (
+	"errors"
 	"math/big"
 
 	"github.com/ethereum/eth-go/ethstate"
@@ -18,9 +19,28 @@ type Environment interface {
 	Difficulty() *big.Int
 	BlockHash() []byte
 	GasLimit() *big.Int
+	Transfer(from, to Account, amount *big.Int) error
 }
 
 type Object interface {
 	GetStorage(key *big.Int) *ethutil.Value
 	SetStorage(key *big.Int, value *ethutil.Value)
 }
+
+type Account interface {
+	SubBalance(amount *big.Int)
+	AddBalance(amount *big.Int)
+	Balance() *big.Int
+}
+
+// generic transfer method
+func Transfer(from, to Account, amount *big.Int) error {
+	if from.Balance().Cmp(amount) < 0 {
+		return errors.New("Insufficient balance in account")
+	}
+
+	from.SubBalance(amount)
+	to.AddBalance(amount)
+
+	return nil
+}